JP3025042B2 - Manufacturing method of ultra-low carbon steel - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing ultra-low carbon steel using a refining device under reduced pressure.

[0002]

2. Description of the Related Art Conventionally, methods for producing ultra-low carbon steel include:
Decarburization in molten steel has been performed by blowing an inert gas into undeoxidized molten steel using a RH reflux type vacuum degassing apparatus and circulating the molten steel in a vacuum chamber. In the treatment of such molten steel, emphasis is placed on increasing the diameter of the immersion pipe and increasing the amount of molten steel recirculated by increasing the circulating Ar gas flow rate to increase the decarburization reaction efficiency, and increasing the stirring power of the molten steel. As an example, a vacuum degassing apparatus described in Japanese Patent Application Laid-Open No. 57-110611 is known. This device is located at the bottom of the vacuum chamber 8 as shown in FIG.
A, 9B is a device provided with a gas jet port 11 at a position between the open ends of the tanks.
Under the condition increased to 00 mm, 50
By blowing 0 to 2000 Nl / min of an inert gas such as Ar into the molten steel, the molten steel 2 is promoted into droplets, the area exposed in the vacuum chamber is increased, and the neutral or oxidizing flux is further reduced. It is intended to effectively promote decarburization and the like by adding it. In the figure, 1 is a ladle,
Reference numeral 10 denotes an inert gas ejection port.

[0003]

However, in the above-mentioned conventional apparatus for producing ultra-low carbon steel, in order to increase the decarburization efficiency (specifically, it is necessary to shorten the decarburization treatment time and to reach the ultra-low carbon steel). The method employs a method of increasing the flow rate of gas blown from the gas jet port 11 under the condition of increasing the amount of molten steel in the vacuum chamber to increase the reaction region of molten steel. However, in this method, there is an upper limit to the flow rate of the blown gas for promoting decarburization. If the flow rate exceeds the upper limit, gas blow-through occurs, and conversely, the reaction region with molten steel decreases. As described above, in the method of blowing the inert gas from the bottom of the (refluxed) molten steel and from the opening end, there is a limit to the improvement of the decarburization efficiency because there is an upper limit of the gas flow rate that does not cause gas blow-through, and the upper part from the vacuum tank bottom. Since the gas is blown toward (in the direction of the exhaust port), the metal scatters not only in the vacuum tank but also in the gas cooler of the vacuum exhaust device, and the operation becomes practically impossible.

The present invention seeks to advantageously overcome the disadvantages of the prior art as described above.

[0005]

SUMMARY OF THE INVENTION The gist of the present invention is to provide a method for blowing a gas such as an inert gas into molten steel in a ladle combined with a vacuum degassing tank to reduce the pressure in the vacuum degassing tank. At the same time, an inert gas is blown into the undeoxidized molten steel within a range surrounded by a, b, c and d in FIG.
And the like, wherein a powder serving as a CO gas generating nucleus is blown.

That is, according to the present invention, an inert gas is blown from above into molten steel in a ladle installed in a vacuum chamber under a reduced pressure atmosphere to promote the dropletization of molten steel and increase the gas-liquid interface area in the vacuum chamber. And a powder of MgO, CaO or the like is blown to form a CO gas generating nucleus for decarburization to promote the decarburization reaction. The details are described below.

[0007]

In the present invention, so-called ordinary non-deoxidized steel is used as molten steel. First, a ladle containing molten steel is combined with a vacuum degassing tank. In this case, it is assumed that the inert gas and the powder can be blown into the molten steel in parallel with the vacuum degassing process on the apparatus.

First, the inside of the vacuum degassing tank starts to be depressurized and reaches a normal vacuum state. An inert gas is blown into the molten steel substantially in parallel with these pressure reduction processes.

[0009] However, these methods are not limited to simply blowing an inert gas, but require specific conditions.

That is, the flow rate Q of blown gas per ton of molten steel
It is effective to increase the decarburization rate to increase the decarburization rate. However, when the flow rate of the blown gas is increased, the rocking and splashing of the molten steel becomes severe, and the metal scatters not only in the vacuum chamber but also in the gas cooler of the vacuum exhaust device, and the operation becomes substantially impossible. In addition, if the lance immersion depth h is increased, the stirring power is increased even at the same gas flow rate, so that the uniform mixing time of the molten steel in the ladle is shortened, and the decarburization speed in the high carbon concentration region is increased, but the molten steel is splashed. The amount of generation is reduced, and in the so-called ultra-low carbon concentration range of [C] ≦ 30 ppm, the decarburization rate is greatly reduced. In order to improve the decarburization rate in the extremely low carbon concentration region, it is essential to increase the amount of splash. However, in order to prevent molten steel from flowing out from the ladle, for example, as disclosed in Japanese Utility Model Application No. 60-84597, It is desirable to provide a splash prevention lid 7 shown in FIG. 2 (where [C] is the carbon concentration in the molten steel in% by weight).

Therefore, under the condition that the splash prevention lid 7 as shown in FIG. 2 is provided, the lance immersion depth h and the blowing gas flow rate are changed to investigate the decarburization rate, and the upper limit of the blowing gas flow rate and the lance immersion Depth conditions were determined. Figure 1
(A) and (b) are conceptual diagrams. In the figure, 1 is a ladle, 2 is undeoxidized molten steel, 3 is a vacuum interface, 4 is a ladle bottom, 5 is an inert gas and powder injection lance, and 6 is a lance gas and powder injection port. .

Assuming that a distance h from the vacuum interface 3 to the gas inlet 6 of the lance and a distance H from the vacuum interface 3 to the ladle bottom 4 are h / H = −0.117Q + 1.97 or less. It is desirable that the region D in FIG.

The lower limit of the flow rate of the blown gas is required to be a flow rate capable of preventing clogging of the lance nozzle, and is 1.7 Nl / min · ts.
The above is desirable, and the region C in FIG. In order to increase the gas-liquid boundary area, it is desirable to inject the gas into the lance at a shallow position where h / H is small and small. Since the decarburization speed decreases and the treatment time increases, there is an appropriate range for practical operation. Further, when the depth is extremely deep, the erosion of the brick at the ladle bottom 4 becomes severe. In order not to adversely affect the brick, h / H ≦ 0.8 is required. That is, h / H = −0.0208Q + 0.433, h / H in consideration of the decarburization rate required for normal operation.
FIG. 3 below Q determined by = 0.175Q-0.125
The areas B and F are not allowed, and should be specified as the area A in the figure.

The deoxidized molten steel is decarburized under the above operating conditions, and after [C] ≦ 50 ppm, the inert gas blown from the lance 5 is used as a carrier gas and the inert gas and the powder are blown into the lance. New C from Mg6, CaO, etc. from mouth 6
A powder that can be an O gas generation nucleus is blown.

According to the present invention, not only decarburization but also degassing such as dehydrogenation can be improved.

Next, an example according to the present invention and a comparative example are shown in FIG.
And FIG. FIG. 3 shows the decarburization rate in the case of a decarburization treatment time of 20 minutes in an example of molten steel of 300 t / ch. In the region of the last stage of decarburization ([C] ≦ 50 ppm),
This is a comparison between the case where gO is blown and the case where gO is not blown. It is recognized that the reaction efficiency is poor in the B and F regions, and the reaction efficiency is good in the region A. Further, in the case of the mark ● in which MgO is blown, the achieved [C] concentration is lower and the reaction efficiency is lower. Becomes better.

FIG. 4 shows the outflow of molten steel due to excess splash for the same example of molten steel as described above.
From this, it can be seen that in the region D, the molten steel oscillates and splashes are excessive, and there is no significant change in the molten steel oscillates and splashes generated by blowing powder.

[0018]

According to the present invention, by operating under specific conditions, it becomes possible to stably produce ultra-low carbon steel with [C] ≦ 25 ppm in molten steel.

[Brief description of the drawings]

FIGS. 1 (a) and 1 (b) are explanatory views of a limiting condition of the present invention,

FIG. 2 is an explanatory diagram of an embodiment of the present invention,

FIG. 3 is a diagram showing an embodiment of the present invention;

FIG. 4 is a diagram showing an embodiment of the present invention;

FIG. 5 is an explanatory view of an embodiment of a conventional method.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Ladle 2 ... Undeoxidized molten steel 3 ... Vacuum interface 4 ... Ladle bottom 5 ... Inert gas and powder blowing lance 6 ... Outlet of inert gas and powder blowing lance 7 ... Splash prevention lid 8 ... RH vacuum chamber 9A: suction pipe 9B: downcomer pipe 10: inert gas jet port 11: inert gas jet port

──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Muneyasu Nasu 1 Kimitsu, Kimitsu City Inside Nippon Steel Corporation Kimitsu Works (72) Inventor Shigeaki Ogibayashi 1 Kimitsu City, Kimitsu City Nippon Steel Corporation Kimitsu (56) References JP-A-58-22319 (JP, A) JP-A-1-176018 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C21C 7/072 C21C 7/10

Claims (1)

(57) [Claims] 1. A method of blowing a gas such as an inert gas into molten steel in a ladle combined with a vacuum degassing tank to reduce the pressure in the vacuum degassing tank.
(B) A decarburization treatment is performed by blowing an inert gas into the undeoxidized molten steel within a range surrounded by a, b, c, and d, and a CO gas such as MgO, CaO, etc. in a region where the carbon concentration in the molten steel is 50 ppm or less. A method for producing ultra-low carbon steel, comprising blowing a powder as a generation nucleus to promote decarburization.